Whenever the ponies are shown front-on their faces look different then when they are shown side-on, they are much flatter front-on and their circular shape does not match what they look like side-on. This is corrected in the movie.
For some reason when a pony with a square-shaped snout is shown front-on their faces are the circular shapes, the same as those with rounded snouts. There are a few exceptions in Season one where Celestia is drawn with the correct shaped snout when facing forward.
Due to an animation error, a background pony was cross eyed in one particular episode and was noticed by the fan base. Writers of the show owned up to the error and instead of correcting it, added a new character based on the error, and hence Derpy Hooves pony was added to the cast!